1. Field of the Invention
The present invention relates to an ultrasonic inspection system and ultrasonic inspection method based on an immersion technique for performing inspection with a liquid such as water interposed between an ultrasonic sensor and an test object.
2. Description of the Related Art
A welded part of a structure has a possibility that various defects may occur therein. As a nondestructive inspection technique to be applied for the interior of the welded part or the internal surface thereof that is inaccessible to a person or an apparatus, an ultrasonic inspection method has been widely adopted. When the surface of the welded part has irregularities or an ultrasonic sensor cannot be brought into direct contact with the surface of the welded part because of the narrowness, ultrasonic inspection based on an immersion technique of separating the ultrasonic sensor from the test object by a certain space and filling the space with a liquid such as water or oil is adopted.
For the conventional ultrasonic inspection based on the immersion technique, an ultrasonic inspection system has been proposed. As described in patent document 1 (JP-A-2005-300363), the ultrasonic inspection system includes a computer-aided design (CAD) device that displays the position of a probe, an incident direction of an ultrasonic wave, and a trajectory of the ultrasonic wave on the basis of information on the shape of a inspection surface, a scanner that positions the probe by making three-dimensional movements, the probe, and a distance sensor that measures a distance from the inspection surface.
According to the proposed ultrasonic inspection system, even if the inspection surface has irregularities or a crack, an optimal probe position is detected and the probe can be moved to the optimal position by the scanner.
Patent document 2 (JP-A-2005-43139) has disclosed a laser ultrasonic inspection apparatus in which a laser light source 1 is used to cause an ultrasonic wave to occur in a member to be inspected, and a laser light source 2 and a light condenser are used to optically detect an ultrasonic signal.
Patent document 3 (JP-A-2009-222642) has disclosed an ultrasonic inspection apparatus that includes an ultrasonic probe and a charge-coupled device (CCD) camera and uses the CCD camera to check an ultrasonic wave incident position.
Further, patent document 4 (JP-A-10-19858) has disclosed an ultrasonic inspection apparatus that records ultrasonic images of an test object and appearance information acquired by a digital camera.
For assessment of the integrity of a structure, an ultrasonic inspection method is widely adopted as a nondestructive inspection technique for the surface of the structure or the interior thereof. When the surface of the structure that is test object has irregularities or curvature or when an ultrasonic sensor cannot be brought into direct contact with the structure because of the narrowness of a space near the structure, such ultrasonic inspection is adopted that: the space between the ultrasonic sensor and test object is filled with a medium (couplant) such as a liquid or a gas which permits propagation of an ultrasonic wave; and the structure is inspected via the medium. For example, when water that is a liquid is used as the medium, the ultrasonic inspection is called an immersion technique. When air that is a gas is adopted as the medium, the ultrasonic inspection is called an airborne ultrasonic wave flow detection method. Thus, the ultrasonic inspection is called differently depending on the medium.
A technique of implementing ultrasonic inspection via a medium is such that inspection is performed by separating an test object from an ultrasonic sensor by a predetermined distance (for example, about several centimeters in the case of the immersion technique) as multiply-reflected waves of an ultrasonic wave may affect a inspection signal in the medium. Therefore, the position of the ultrasonic sensor and a point (ultrasonic wave incident position) on an test object which the ultrasonic wave incidents after propagating through the medium have a spatial distance between them. In order to obtain a more highly reliable result of inspection, it is necessary to accurately grasp the positional relationship between the ultrasonic sensor and test object, or more particularly, the ultrasonic wave incident position on the test object.
For example, in relation to the conventional ultrasonic inspection, an ultrasonic inspection apparatus combined with imaging equipment has been disclosed in patent document 5 (JP-A-2010-32434). Herein, a method implemented as a conventional method of identifying an ultrasonic wave incident position is such that: an image of an object of testing is picked up using a video camera; reflection of an ultrasonic wave is detected while coordinates representing the position of an ultrasonic probe are measured using the camera; and a inspection image obtained based on an ultrasonic reflected wave and a camera image are displayed while being superposed on each other.
Further, patent document 6 (JP-A-6-102258) has disclosed an ultrasonic inspection method and system in which: a laser or an LED attached to an ultrasonic probe is imaged using a camera in order to find out a three-dimensional shape of an object of inspection; and a three-dimensional graphic image is produced and displayed while being superposed on an ultrasonic inspection screen image.
The foregoing techniques have been devised on the assumption that a direct contact technique of bringing an ultrasonic sensor into contact with an test object is adopted. The patent documents describe the methods, apparatuses, and systems that identify the position of the ultrasonic sensor. However, as long as an immersion technique or any other ultrasonic inspection method in which the position of the ultrasonic sensor and an ultrasonic wave incident position are inconsistent with each other and a medium is employed is adopted, the incident position cannot be identified.